Preheater for diffusion pump



July 9, 1968 LUCAS ET AL 3,391,857

PREHEATER FOR DIFFUSION PUMP Filed Sept. 1, 1966 INVENTORS LUTHER R. LUCAS BY RICHARD M. REIMERS ATTORNEY United States Patent 3,391,857 PREHEATER FDR DIFFUSION PUMP Luther R. Lucas, Palo Alto, and Richard M. Reimers,

Oakland, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Sept. 1, 1966, Ser. No. 577,118 7 Claims. (Cl. 230-101) ABSTRACT OF 'I'HE DISCLOSURE A high vacuum diffusion pump having valves for isolating the boiler from the vapor jet forming structure during a warm up period and having means for using vapor from the boiler to preheat the jet forming structure so that the jets function efiiciently and without bacle streaming immediately after being turned on.

This invention relates to high vacuum pumps and more particularly to a diffusion pump which may remain in communication with the vessel to be evacuated during start-up and shut-down periods without causing the release of pumping vapor into the vacuum. The invention described herein was made in the course of, or under, Contract W-7405-eng-48 with the United States Atomic Energy Commission.

Difiusion pumps, which are widely used for producing high vacuum, have a series of vapor jets, fed by a pumping fluid boiler, which entrain and withdraw gas particles from the region to be evacuated. Difliculties have been experienced as a result of the initially cold condition of these pumps. Upon starting the pump, it is essential that the interior of the pump and the vessel to be evacuated first be pumped down to a relatively low pressure by another form of vacuum pump known as a fore-pump. Following this pump-down, the pumping fluid must be heated sufficiently to start the formation of vapor to operate the jets. Heretofore it has been necessary to operate the pump for some time to thoroughly heat up the jet nozzle structure and the tubulation which conducts vapor to the nozzles before satisfactory pumping operation could be obtained. Rapid condensation of vapor on the surfaces of the nozzle structure, which could release pumping fluid vapor into the vacuum vessel, is one cause of the difliculties which occur when the pump structure is still cold.

Owing to the factors discussed above, it has heretofore been necessary to isolate the diffusion pump from the system to be evacuated by means of a vacuum valve, while the pump is shut-down and during start-up, until it is completely warmed-up and working properly. The vacuum valve is cumbersome, costly, and inconvenient, and reduces the pumping speed of the system.

The present invention is a diffusion pump in which the vapor nozzle structure and tubulation conducting vapor to the nozzles may be preheated by means of vapor from the boiler thus preparing the pump for efiicient operation before vapor is provided to the nozzles to start the pumping operation. In order to preheat the boiler and use vapor therefrom to heat the nozzle and tubulation structures prior to the provision of vapor to the jets, it is necessary to isolate the boiler from the nozzle system and pumping cavity and valves are provided for this purpose. As a result of the valve placed between the boiler and the jet structure and a second valve closing off the condensate return path, the body of the pump exclusive of the boiler may be maintained at vacuum pressure. In the case of an individual pump this is forevac pressure and of course the remainder of the vacuum would also be maintained at this pressure. In the case of a multi-pump system this "ice is at high vacuum pressure. In the multi-pump case, a valve is required in the line from the pump to the forevac system but is considerably smaller and simpler than a valve on the input side of the pump.

Accordingly it is an object of the present invention to provide a diffusion pump which need not be isolated from the associated vacuum vessel during start-up and shut-down periods.

It is a further object of this invention to provide means for isolating the boiler of a diffusion pump from the jet structure when required in operation so that the boiler may be heated and the jet nozzle structure may be preheated before vapor is permitted to pass through the jets.

It is another object of the invention to provide means to use vapor from the boiler to preheat the jet nozzle structure of a diffusion pump prior to putting the pump into operation.

The invention and its further advantages and objects thereof will be better understood by reference to the following specification in conjunction with the accompanying drawing which is an elevation view of a diffusion pump, partly in section and partly cut-away, and which illustrates one embodiment of the invention.

Referring now to the drawing the pump has a boiler 11 which is cylindrical in shape and is composed of a circular top 12 and bottom 13 and a cylindrical side wall 14 which has a height about one third of its diameter. A tube 17 extends upwardly from a central bore 16 in the top 12. Inside tube 17, coaxial therewith and spaced apart therefrom, is a second tube 18 which delivers heated vapor from the boiler 11 to a series of pumping jets 24. Tube 18 is mounted by means of a toroidal member 19 which seals the two tubes together at their tops. The inner tube 18 extends downward to approximately the level of the lower side of top 12. This construction leaves the space between tubes 17 and 18 open to vapor from the boiler 11. A cruciform bracket 21, the four ends of which are mounted on toroidal member 19, supports the upper portion 22 of the tubular structure 23 which conducts vapor from the upper end of tube 18 to the jet nozzles 24 two of which are below the upper end of this tube. For these nozzles 24 the vapor is conducted downwardly between the outer surface of tube 17 and the tubular structure 23. Below the lowest of the jet nozzles 24 a ring 26 seals the lower end of the passage between the outer tube 17 and tubular structure 23. This ring 26 also serves to conduct heat from the tube 17 to the tubular structure 23. This heat conduction from tube 17 to tubular structure 23 is augmented by the spacers 25 placed between the tube and the structure just above the middle set of nozzles 24 and also by the bracket 21.

A cylindrical barrel 27, mounted coaxially around tubular structure 23 and spaced outwardly from the jet nozzles 24, is interposed between the jet nozzles and the outer shell 30 of the diffusion pump, forming a Wall of the pumping compartment. A flat annular member 28 sealed at its outer edge to shell 30 and at its inner edge to the tubular structure 23 forms the bottom of the pumping chamber.

To return condensed pumping fluid vapor from the pumping chamber to boiler 11, a tube 29 transpiercing the bottom plate 28 leads downwardly through a remotely controlled valve 35 and thence up through the bottom 13 of the boiler in sealed relationship thereto. An outwardly extending flange 32 at the top of shell 30 provides means for connecting the pump to the vessel to be evacuated. Tubing 33 coiled around the outside of barrel 27 provides for the circulation of cooling fluid for the pumping compartment wall. Tubulation 34 penetrating the side of shell 30 near the top thereof has a flange 36 at its outer end for connection to the forevac line of a forevacuum pump.

Most of the outer area of the jet nozzle supporting structure 23 is covered by insulation 37 to aid in keeping the barrel 27 of the pump cool and keeping the nozzles 24, tubular structure 23 and tubes 17 and 18 Warm to the extent required for best operation.

A tube 38 approximately the same diameter as tube 17 extends downwardly from a bore 3? in the bottom plate 13 of the boiler 11. The tube 38 is coaxial with tube 17 and has a flange 4 2* at the lower end. A tube 42 of somewhat larger diameter than tube 38 extending downwardly therefrom has a flange 41 thereon which is coupled to flange 40 in a sealed joint. The tube .2 is closed at its lower end by a circular plate 43 except for an air pressure fitting 44 secured in the plate to penetrate therethrough.

Inside the tube 42 a tubular bellows 46 has its upper end secured to flange 41. A tube 47 small enough in outer diameter to move freely within the tube 38 extends down through tube 38 and down through the tubular bellows 46 and is sealed to the lower end of the bellows by means of an annular member 48 having a U-shaped cross section. This connection of tube 47 to bellows 46 will normally hold the tube in its downward position when no external force is applied. The tube 47 is closed at the top by a valve element 49 generally cylindrical in shape and of the same outer diameter as the tube. The valve 49 further has a cruciform extension 51 extending from the top thereof, which slides into the inner tube 18 when the valve is closed. The upper corners of the cruciform extension 51 are cut at an angle to aid in guiding the upper and outer edge of the valve element 49 into the end of tube 18. A ring shaped gasket 52 mounted on the upper face of the valve element 49 and encircling the cruciform extension 51 serves to seal the end of tube 18 when element 49 is pushed upward by means of air pressure introduced through the fitting 44. Therefore the element 49 controls the flow of vapor from the boiler 11 to the vapor nozzles 24 and may be remotely operated by controlling the air pressure supplied to the fitting 44. The valve element 49 together with the seat formed by the end of tube 18 comprise the valve 53.

Electrical heaters 54 secured in the side 14 of boiler 11 are provided to heat the pump liquid 31 and thus produce vapor.

In operation when a vacuum system is to be evacuated and connection to the system has been made by means of flange 32, the valve 53 is closed by applying compressed air to fitting 44 and valve 35 is also closed. The forepump connected to flange 36 then is started to produce the initial vacuum required to bring the system within the operating range of the diffusion pump. Heaters 54 are energized at the proper time to heat pump liquid 31 thus creating vapor which rises between tubes 17 and 18 as previously described and thus heats these tubes. Further by means of radiation from tube 17 and by means of conduction through ring 26, spacers 25 and bracket 21 the tubular structure 23 and jet nozzles 24 are also heated until the pump is at the proper temperature for eflicient Operation. This should coincide in time with the attainment of the proper degree of vacuum in the system by means of the forevacuu-rn pump. Shortly before the completion of the Warm-up period refrigerating fluid is provided through tube 33 to cool barrel 27 and this fluid continues to flow during the operation of the pump. The valve 53 is then opened by removing the high air pressure from fitting 44 and the valve 35 is also opened, whereupon vapor is supplied through the inner tube 18 to jet nozzles 24. These nozzles 24 and the tubulation 18 and 23 leading thereto, having been previously heated, will produce eflicient pumping action immediately by the passage of the vapor therethrough.

Where a plurality of pumps are used to evacuate a large system, the shut-down of a particular pump may be desired while leaving other parallel pumps to maintain the vacuum. To accomplish this, valves 53 and 35 are closed and the flow of coolant through the tube 33 terminated. In this operation it is also necessary to provide a valve in the tube leading to the forevacuum pump which is connected to the flange 36, since the nozzle structure 24 and pumping space around the nozzles remain at high vacuum under the action of other pumps. This valve may be much smaller and simpler than any valve placed at the input to the pump. The heaters 54 are then turned off. To place the pump back in service a reverse of this procedure is followed. When the invention is used in service of the type just described it may be desirable to evacuate the upper portion of the boiler 11 and the space between the inner tube 18 and outer tube 17 prior to the generation of vapor in the boiler. Thermostatic control of the boiler heaters 54 may be provided if desired.

Although the invention has been disclosed with respect to a single exemplary embodiment it Will be evident to those skilled in the art that many variations are possible within the spirit and scope of the invention. Therefore it is not intended to limit the invention except as defined by the following claims.

What is claimed is:

1. In a diffusion pump, the combination comprising a barrel forming a pumping chamber and having an inlet and outlet, a jet forming tubulation extending Within said barrel and having at least one nozzle for establishing a vapor jet directed towards the inner wall of said barrel and away from said inlet, a pumping fluid boiler having vaporizable liquid therein and having heating means associated therewith, a valve connecting said boiler to said jet forming tubulation whereby the vapor passage therebetween may be selectively opened and closed, and a heat transmitting means having a vapor conduit opening only into said boiler and extending into said jet forming tubulation independently of said valve whereby heat from said vapor heats said jet forming tubulation while said valve is in the closed position.

2. A diffusion pump as described in claim 1 having a plurality of said jet nozzles and wherein said jet forming tubulation has a vapor supply tube extending therewithin from the upper portion of said boiler to the region of the uppermost of said nozzles, said heat transmitting means being disposed coaxially around said tube.

3. A diffusion pump as described in claim 1 wherein said heat transmitting means extends coaxially within said jet forming tubulation and wherein a plurality of heat conducting elements extend from said heat transmitting means to said tubulation.

4. A diffusion pump as described in claim 1 wherein said barrel is connected to said boiler by a return conduit for pumping vapor condensed within said barrel and comprising the further combination of a valve in said conduit to permit further isolation of said boiler from said pump barrel when said valve to said jet forming tubulation is closed.

5. A diffusion pump as described in claim 1 wherein a tube extends upwardly from said valve into said jet forming tubulation to supply said vapor thereto from said boiler, and wherein said heat transmitting means comprises a second tube disposed coaxially around said first tube in spaced relation therefrom, said second tube being closed at the upper end and having an open lower end communicated with said boiler.

6. A diffusion pump as described in claim 1 wherein said vapor is supplied to all nozzles of said jet forming tubulation through a tube extending upwardly therethrough from the upper portion of said boiler, and wherein said valve is comprised of a movable closure member disposed in said boiler and seatable in the lower end of said tube, and control means extending into said boiler for operating said closure.

7. A diffusion pump as described in claim 6 wherein said control means is comprised of a collapsible bellows having a first open end secured to said boiler at an opening in the underside thereof, the second end of said bel- 5 6 lows being closed, an element disposed within said bel- 2,404,021 7/1946 Alexander et 211 230-1()1 lows and connecting said closure member to said second 2,438,395 3/1948 Hopper 230-101 end thereof, a fluid tight enclosure surrounding said 'bel- 2,608,343 8/1952 Colaiaco et al. 230101 lows, and means for selectively admitting a pressurized 2,797,043 6/1957 Gerow 230101 X fiuid into said enclosure whereby said bellows is corn- 5 2,899,127 8/1959 Power 230-101 pressed thereby lifting said closure member to said lower 3,344,979 10/1967 Chester 230--101 end of said vapor supply tube.

FRED C. MATTERN, 111., Primary Examiner. References Cited UNITED STATES PATENTS 1U DONLEY J. STOk/KING, Exammer.

2,l50,685 3/1939 Hickman 23() 1()1 W. I, KRAUSS, Assismnt Examiner. 2,386,298 10/1945 Downing et a1. 230101 

